Do all sunglasses block UV? My friend just told me polycarbonate (what he's calling plastic) blocks UV by default, so there's no point paying for expensive UV-blocking sunglasses.
How right is he?
 A: He's right about not needing to pay for expensive sunglasses for UV protection.
Polycarbonate does have properties that block UV radiation.  From what I can find, it is very effective.  Not all sunglasses will be made of polycarbonate, for various reasons, and thus may not block UVA/UVB.  Other materials can have properties that block UV radiation, or if required, can be coated to reduce UV transmission.  
If your only purpose is to achieve some level of UV radiation; you can likely find an inexpensive pair of sunglasses which have good UVA and UVB blocking properties; probably made of something like polycarbonate.  Most sunglasses with UV resistance have some sort of indication about which standards they meet; which will determine how much UV the material is designed to block.
The important thing to consider here is that not all sunglasses are designed for UV protection.  There's a large market for sunglasses which are made more for style than for practicality; so features like UV protection may be lacking in favour of other design choices.  If we were only paying for UV protection, sunglasses would probably be fairly cheap.
